According to the structure, golf balls are classified into many types. For example, one-piece golf balls have a single structure made from a homogeneous mass of material, two-piece golf balls of a double structure consisting of solid matter in which a core is surrounded by a cover, solid type three-piece golf balls of a triple structure with a dual core consisting of an inner core and an outer layer covering the said inner core and a cover covering the said dual core, solid type three-piece golf balls of a triple strucure comprising a core and double cover in which an inner cover covering the said core and an outer cover covering the said inner cover, rubber thread wound three-piece golf balls of a triple structure comprising a rubber thread wound liquid or solid core is surrounded by a cover, four-piece golf balls of a quadruple structure comprising a triple core which is surrounded by a cover, four-piece golf balls of a quadruple structure comprising a dual core and double cover in which an inner cover covering the said dual core and an outer cover covering the said inner cover, four-piece golf balls of a quadruple structure comprising a core and triple cover which covered the said core, etc., golf balls have got more and more developed in many structures and variety of types in actuality. The reason why many types of golf balls are prepared as mentioned above, is said to be attributed to the fact that there have not yet been developed golf balls with which all golfers can be satisfied. In general, the golf ball which can give a satisfaction to golfers, that is the golf ball fly a great distance in hitting with a driver and excellency in back-spin property in hitting with a short iron, and soft enough to give a good impact feeling to the golfers in addition to being superior in durability.
When the golfer hit a golf ball with a golf club, a strong repulsive elasticity is generated on the ball by the impact force applied from the head of a golf club, bounding the ball at a high speed and thus making a carry distance with the aid of the back-spin due to the loft angle of the club head and the aerodynamic lifting force due to the dimples on the surface of the golf ball.
The important factors which determine the repulsive elasticity of golf ball are materials, their compositions, and the preparation methods for making the golf ball. The repulsive elasticity is much more affected by cores, such as single core, dual core, triple core, etc., than by covers in the solid type golf balls.
Likewise, rubber thread wound type golf ball's repulsive elasticity is more affected by a liquid center or solid core and the rubber threads wound around them than by covers. However, the cover or double cover of a golf ball plays a critical role in obtaining the back-spin needed when hitting a golf ball with a short iron club, in particular, the materials of the covers may determine the ball's function. Hereinafter, covers, double covers, triple covers are represented simply by covers. The materials of the covers of golf balls on the market are usually made from polymeric resins, such as ionomer resins, Balata rubber, polyurethane, polyetherester, polystyrene-polybutadiene block copolymer, other thermoplastics, or the mixtures thereof, or in combination with silicone resins, fluorine resins, etc. Among them, ionomer resins are the most frequently used. The ionomer resin covers show strong physical properties, but it is difficult to make a good golf ball by the reason of too hard to give a good impact feeling and the poor back-spin property in short iron shot.
In order to solve the problems, modified soft ionomer resins were developed and applied to many golf balls, however the covers made from the soft ionomer resins have so many problems, such as the surface of cover is easily damaged in hitting or significantly deteriorated the repulsive elasticity relative to the surface hardness. On the other hand, balata rubber, conventionally used as a cover material for rubber thread wound golf balls, which is superior in repusive elasticity and back-spin property, however the balata cover is poor in durability, the cover is torn or collapsed easily in hitting owing to the weak physical property. As for a polyurethane cover, it is superior in back-spin property, but the golf ball flies a significantly reduced distance in hitting with a driver owing to the poor repulsive elasticity, and the ball becomes too hard in the cold season to give the good feeling for the golfers. In addition, the molding process is too difficult to make a cover from polyurethane, the productivity is disadvantageous. Likewise, the other resins do not give remarkable advantages as cover materials for golf balls, and they have a lot of problems in many aspects.
Technical Assignment to be Achieved in this Invention
It is an object of the present invention to solve the above-mentioned problems of the covers in prior arts and to make a cover composition for golf balls which can fly a longer carry distance in hitting with a driver and have a superior back-spin property in short iron shot, and to provide a cover composition for golf balls which is superior in durability and gives the golfers a soft impact feeling.